The chamber geometry of a system for molecular beam epitaxy (MBE) is known to affect technical aspects such as the macroscopic deposition homogeneity across a wafer. However, for the microscopic mechanisms governing the growth of thin films on a planar substrate, the chamber geometry does not play any role. In marked contrast, we show here that the luminescence efficiency of (In,Ga)As/GaAs shell quantum wells grown around GaAs nanowires changes by more than two orders of magnitude depending on the relative position of the As cell compared to the group-III cells.
The origin of this drastic effect is that for a compound nanowire shell, in MBE the constituent elements are inherently deposited sequentially. More specifically, in MBE deposition takes place only in free line of sight from the cells with source material. Thus, for the growth of compound shells around nanowires standing perpendicularly on the substrate, the molecular beam from a given cell can reach because of self-shadowing by the nanowire core not all of the nanowire sidewall facets at the same time. Substrate rotation leads to subsequent exposure and shadowing of a given sidewall facet. Consequently, the deposition of the constituents is inherently sequential, and the azimuthal arrangement of the cells in the MBE chamber is decisive for the deposition sequence. We found that a high photoluminescence intensity is obtained only for a flux sequence corresponding to migration enhanced epitaxy (MEE), i. e. when As and the group-III metals essentially do not impinge at the same time. A detailed analysis showed that for co-deposition, the luminescence efficiency is dramatically reduced because of the increased incorporation of both shallow and deep point defects.
Our sample structure is a model system for optically active nanowire shells, and we expect that our findings generally hold for compound nanowire shells. We emphasize that the azimuthal cell arrangement is in this context of extraordinary relevance, but difficult to change between growth runs.
Author: Hanno Küpers , Ryan B. Lewis , Pierre Corfdir , Michael Niehle , T. Flissikowski , H. T. Grahn , A. Trampert , O. Brandt , L. Geelhaar
Title: Drastic effect of sequential deposition resulting from flux directionality for the luminescence efficiency of nanowire shells
Source: ACS Appl. Mater. Interfaces , 13 , 50220 ( 2021 )